Rendering multicast service with sufficient reception quality to wireless terminals

ABSTRACT

The method of invention provides a multicast service from an information delivery apparatus to wireless terminals through wireless routes. The method includes the step of transmitting, from the information delivery apparatus, a plurality of sets of multicast information, wherein these sets are identical to each other as to contents thereof but differ in transmission conditions. The method further includes the step of receiving, at any given one of the wireless terminals, one of the sets of multicast information being transmitted under one of the differing transmission conditions.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a method of providingmulticast services, and particularly relates to a method of providingmulticast services by having an information delivery apparatus delivermulticast information to wireless terminals residing in a service areathrough wireless routes. The present invention further relates to aninformation delivery apparatus and a wireless terminal used in such amethod of providing multicast services.

[0003] 2. Description of the Related Art

[0004] In recent years, applications that deliver music and videocontents through the Internet have been attracting much attention. Useof wireless communication as an access link has an advantage in thatinformation delivery services can be easily provided to users. If musicor video contents are delivered as part of such services through anetwork that includes wireless routes as an access link, resultingmerits are enormous.

[0005] In a general configuration of multicast service delivery systemsthat deliver multicast information such as music and video contentsthrough wireless routes, a wireless base station (i.e., informationdelivery apparatus) transmits multicast information to a plurality ofwireless terminals, and these wireless terminals simultaneously receivethe multicast information. If the system is configured such thatone-to-one communication is carried out between the wireless basestation and each of the wireless terminals, the same information havingthe same contents needs to be delivered through separate channels toindividual wireless terminals that are requesting the service. Such aconfiguration cannot make efficient use of communication resources.

[0006] The same multicast information is thus transmitted at once to aplurality of wireless terminals as part of the delivery service. In sucha case, each wireless terminal experiences different quality ofreception when receiving the multicast information if the wirelessterminals are situated in different wireless communication environments.As shown in FIG.17, for example, wireless terminals H and J that arerelatively closer to the wireless base station BS are situated inrelatively better communication environments, so that quality ofreception is relatively high when receiving multicast information fromthe wireless base station BS. Since wireless terminals F, G, and I thatare situated farther away from the wireless base station BS than thewireless terminals H and J experience relatively poor communicationconditions, quality of reception would be poorer. Further, wirelessterminals A, B, C, D, and E that are positioned close to a service areaEs far away from the wireless base station BS suffer much worsecommunication environment, so that quality of reception of receivingmulticast information from the wireless base station BS would be muchpoorer.

[0007] When multicast services are rendered by using one-to-n wirelesscommunication between the information delivery apparatus and eachwireless terminal as described above, each wireless terminal receivingthe multicast services may differ in its reception quality. If receptionquality varies, there may be a situation in which too many errors aremade during reception so as to fail to meet required reception quality.

[0008] Accordingly, there is a need for a method of rendering multicastservices that can deliver multicast information to wireless terminalsexperiencing different reception conditions in such a manner as toachieve sufficient reception quality at each of the wireless terminal.

[0009] Further, there is a need for an information delivery apparatusthat is suitable for such a method of rendering multicast services.

[0010] Moreover, there is a need for a wireless terminal that issuitable for the method of rendering multicast services.

SUMMARY OF THE INVENTION

[0011] It is a general object of the present invention to provide ascheme of providing multicast services that substantially obviates oneor more of the problems caused by the limitations and disadvantages ofthe related art.

[0012] It is another and more specific object of the present inventionto provide a method of rendering multicast services that can delivermulticast information to wireless terminals experiencing differentreception conditions in such a manner as to achieve sufficient receptionquality at each of the wireless terminal.

[0013] It is a further object of the present invention to provide aninformation delivery apparatus an a wireless terminal that are suitablefor practicing such a method of rendering multicast services.

[0014] In order to achieve the above objects according to the presentinvention, a method of providing a multicast service from an informationdelivery apparatus to wireless terminals through wireless routesincludes the steps of transmitting, from the information deliveryapparatus, a plurality of sets of multicast information, said sets beingidentical to each other as to contents thereof but differing intransmission conditions, and receiving, at any given one of the wirelessterminals, one of the sets of multicast information being transmittedunder one of the differing transmission conditions.

[0015] In the method of providing multicast services described above,since the information delivery apparatus transmits the plurality of setsof multicast information having the same contents but varying in thetransmission conditions thereof, a given wireless terminal residing inthe service area of the information delivery apparatus can receive oneof the sets of multicast information delivered under such a transmissioncondition as suitable for the reception quality of the given wirelessterminal.

[0016] These transmission conditions are defined for the transmission ofmulticast information to the wireless terminals, and are of such anature as affecting the reception quality of each wireless terminal.

[0017] According to another aspect of the present invention, the methodas described above is such that the differing transmission conditionsinclude differing transmission rates at which the multicast informationis transmitted.

[0018] In the method of providing multicast services described above, awireless terminal that cannot attain sufficient reception quality at ahigh transmission rate can choose to receive the multicast informationdelivered at a low transmission rate.

[0019] According to another aspect of the present invention, the methodas described above is such that communication between the informationdelivery apparatus and the wireless terminal is based on code divisionmultiple access, and the differing transmission rates differ in a numberof spreading codes used in the transmission of multicast information.

[0020] If communication between the information delivery apparatus andthe wireless terminal is based on time division multiple access, thediffering transmission rates differ in a number of timeslots used in thetransmission of multicast information.

[0021] Alternatively, the differing transmission rates differ in thenumber of modulation levels used for modulating the multicastinformation.

[0022] Alternatively, the differing transmission rates differ in atransmission bit rate of the multicast information.

[0023] Alternatively, if communication between the information deliveryapparatus and the wireless terminal is based on code division multipleaccess, the differing transmission conditions include differingprocessing gains of spreading the multicast information.

[0024] In order to avoid co-channel interference, the method asdescribed above is such that the differing transmission conditionsinclude differing positions of timeslots used in the transmission ofmulticast information.

[0025] In the method of providing multicast services described above,the level of the co-channel interference differs depending on thepositions of timeslots, so that a given wireless terminal can receivethe multicast information by using proper timeslots that are selected inresponse to the reception quality of the terminal to provide the lowestco-channel interference.

[0026] In order that the information delivery apparatus can deliver themulticast information by using transmission conditions suitable to eachwireless terminal's reception quality, the method as described abovefurther includes the steps of measuring reception quality at each of thewireless terminals, and notifying the information delivery apparatus ofmeasured results of the reception quality, and determining, at theinformation delivery apparatus, the differing transmission conditionsbased on the measured results of the reception quality, the differingtransmission conditions being used to transmit the plurality of sets ofmulticast information.

[0027] In order that each wireless terminal can easily receive themulticast information delivered by using transmission conditionssuitable for each wireless terminal's reception conditions, the methodas described above further includes a step of having the informationdelivery apparatus notify the wireless terminals of the differingtransmission conditions, wherein said step of receiving receives the oneof the sets of multicast information by using the one of the differingtransmission conditions that is notified by the information deliveryapparatus.

[0028] In order that each wireless terminal can actively select andreceive the multicast information at the better reception quality, themethod as described above further includes the steps of reporting, fromthe information delivery apparatus to the wireless terminals, thediffering transmission conditions used to transmit the plurality of setsof multicast information, and measuring reception quality at each of thewireless terminals, and selecting a transmission condition from thereported differing transmission conditions based on the measuredreception quality, the selected transmission condition being used forreceiving one of the sets of multicast information.

[0029] In order to achieve a substantially equal delivery time for eachtransmission rate when delivering the same multicast information atdifferent transmission rates, the method as described above furtherincludes a step of decreasing a size of the multicast information to betransmitted as the differing transmission rates decrease.

[0030] According to another aspect of the present invention, the methodas described above is such that said step of decreasing adjusts acompression rate of the multicast information to be transmitted so as todecrease the size of the multicast information.

[0031] In order to deliver the multicast information at differenttransmission rates by reading it from a single buffer, the method asdescribed above further includes the steps of storing the multicastinformation in a buffer at the information delivery apparatus as themulticast information is received from a network, and assigning channelsto the respective sets of the multicast information as the respectivesets are read from the buffer at rates of reading corresponding to thediffering transmission rates.

[0032] In order to eliminate difficulties caused by delays ofinformation delivery as these delays are generated by differences intransmission rates when delivering the multicast information atdifferent transmission rates from a single buffer, the method asdescribed above further includes a step of adjusting the differingtransmission rates based on delays of the reading of the multicastinformation from the buffer.

[0033] Further, according to the present invention, an informationdelivery apparatus for delivering multicast information to wirelessterminals through wireless routes includes a multicast informationstorage unit which stores the multicast information to be transmitted,and an information delivery control unit which transmits a plurality ofsets of the multicast information, which are identical to each other asto contents thereof but differ in transmission conditions.

[0034] Moreover, a wireless terminal for receiving multicast informationfrom an information delivery apparatus through wireless routes includesa control unit which measures reception quality of signals received fromthe information delivery apparatus, and receives one of sets of themulticast information sent from the information delivery apparatus byusing transmission conditions selected based on the measured receptionquality, wherein the sets of multicast information are identical to eachother but differ in transmission conditions.

[0035] Other objects and further features of the present invention willbe apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is an illustrative drawing showing an example of a systemfor providing multicast services according to an embodiment of thepresent invention;

[0037]FIG. 2 is a block diagram of an example of a wireless basestation;

[0038]FIG. 3 is a block diagram showing an example of a wirelessterminal;

[0039]FIG. 4 is an illustrative drawing for showing the way themulticast information is transmitted through separate transmissionchannels to wireless terminals situated different communicationenvironments by controlling a processing gain in response to a requiredtransmission rate;

[0040]FIG. 5 is a chart showing differences in transmission time betweena low rate transmission and a high rate transmission;

[0041]FIG. 6 is a chart showing relationships in the frequency domainbetween a bandwidth after spreading, a bandwidth of high ratetransmission, and a bandwidth of low rate transmission;

[0042]FIG. 7 is an illustrative drawing showing different time slotsthat carry multicast information having different processing gains ofspreading;

[0043]FIG. 8 is an illustrative drawing showing the way the samemulticast information is delivered by changing the number of spreadingcodes;

[0044]FIG. 9 is an illustrative drawing showing different timeslots thatcarry multicast information spread by different spreading codes;

[0045]FIG. 10 is a table showing reception qualities and correspondingtransmission rates;

[0046]FIG. 11 is an illustrative drawing showing the way the samemulticast information is transmitted by changing the position oftimeslots;

[0047]FIGS. 12A through 12C are illustrative drawings showing deliveryof multicast information by using varying timeslot positions;

[0048]FIG. 13 is an illustrative drawing showing the way the samemulticast information is transmitted by changing the number ofmodulation levels;

[0049]FIG. 14 is an illustrative drawing showing different time slotsthat carry multicast information modulated by different modulationschemes;

[0050]FIG. 15 is a table showing reception qualities and correspondingtransmission rates;

[0051]FIG. 16 is an illustrative drawing showing buffer pointers usedwhen transmitting the same multicast information at differenttransmission rates; and

[0052]FIG. 17 is an illustrative drawing for explaining a case in whichwireless terminals situated in different wireless communicationenvironments experience different quality of reception when receivingmulticast information.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] In the following, embodiments of the present invention will bedescribed with reference to the accompanying drawings.

[0054]FIG. 1 is an illustrative drawing showing an example of a systemfor providing multicast services according to an embodiment of thepresent invention.

[0055] In FIG. 1, a wireless base station 20 serving as an informationdelivery apparatus covers a service area Es. The wireless base station20 obtains information to be delivered through a network NW (e.g., an IPnetwork), and attends to delivery service to deliver the obtainedmulticast information to each wireless terminal 10 residing in theservice area Es.

[0056]FIG. 2 is a block diagram of an example of the wireless basestation 20.

[0057] In FIG. 2, the wireless base station 20 includes a transceiver21, a multicast information storage unit 22, a network control unit 23,and an information delivery control unit 24. The transceiver 21 carriesout wireless communication with each wireless terminal 10 residing inthe service area Es. The network control unit 23 serves as a source ofinformation, and communicates with a server through a predeterminednetwork (e.g., an IP network), for example, to obtain multicastinformation to be delivered. The multicast information storage unit 22stores the multicast information that the network control unit 23received through the predetermined network. This corresponds to abuffering operation. The information delivery control unit 24 attends tocontrol for delivering the multicast information from the transceiver 21to each wireless terminal 10 situated in the service area Es as themulticast information is successively stored in the multicastinformation storage unit 22.

[0058]FIG. 3 is a block diagram showing an example of each wirelessterminal 10.

[0059] In FIG. 3, the wireless terminal 10 includes a transceiver 11, anoutput unit 12, and a control unit 13. The transceiver 11 exchangesinformation with the transceiver 21 of the wireless base station 20. Theoutput unit 12 outputs the multicast information (e.g., music or video)in a proper format corresponding to the type of the information (e.g.,audio format or video format) as the transceiver 11 receives themulticast information from the wireless base station 20. The controlunit 13 controls the transceiver 11 and the output unit 12, and measuresreception quality of downlink signals received by the transceiver 11.Such reception quality may include a reception level, an interferencelevel, a noise level, an error rate, etc.

[0060] In the system for rendering multicast services as describedabove, each wireless terminal 10 residing in the service area Es of thewireless base station 20 uses the control unit 13 to measure receptionquality such as a reception level with regard to a free downlink channelthat is used by the transceiver 11 for signal reception. Each wirelessterminal 10 notifies the wireless base station 20 of the results ofmeasurement. Notification of the measured results of reception qualitymay be made by attaching the measured results to a request signal thatis transmitted to the wireless base station 20 for the purpose ofrequesting delivery service of a desired multicast group (variety ofmusic or video, multicast information). If each wireless terminal 10carries out communication of unicast information other than thereception of multicast information with the wireless base station 20(i.e., if unicast is additionally performed), the notification ofmeasured results of reception quality may be made by using a controlchannel associated with the channel for the communication of unicastinformation.

[0061] In response to the request signal requesting a multicast group,the wireless base station 20 transmits to the wireless terminal 10 thetransmission conditions of multicast information corresponding to therequested multicast group. The transmission conditions of multicastinformation specifies requirements for the transmission of multicastinformation to the wireless terminal 10, and may indicate a wirelesschannel, a transmission rate, the number of modulation levels, atransmission timeslot, a processing gain PG of spreading, spreadingcodes, and the number of the spreading codes, etc. Information aboutsuch conditions is necessary for each wireless terminal 10 to receivemulticast information from the wireless base station 20. Theseconditions may affect reception quality when each wireless terminal 10receives multicast information. The wireless base station 20 may be ableto transmit the transmission conditions of multicast information whentransmitting to each wireless terminal 10 a signal responding to therequest signal received from each wireless terminal 10.

[0062] The information delivery control unit 24 of the wireless basestation 20 takes into account the rate of multicast information receivedfrom the network and at leas some of the measured results of receptionquality received from the wireless terminal 10 that requested themulticast information. Based on this, the information delivery controlunit 24 determines how to deliver the requested multicast information.

[0063] In the case of a system based on code division multiple access(CDMA), a change may be made to the processing gain PG of spreading,thereby changing the transmission band after spreading. In general, theprocessing gain PG is raised to improve transmission quality.

[0064]FIG. 4 is an illustrative drawing for showing the way themulticast information is transmitted in a multiplexed manner throughseparate transmission channels to wireless terminals situated differentcommunication environments by controlling the processing gain PG inresponse to a required transmission rate.

[0065] The rate of given multicast information is I, and the bandwidthafter spreading is B. Since wireless terminals A through E in FIG. 4reside in poor communication environments because of weak receptionsignals or the like, required reception quality is not satisfied unlessa low rate transmission (fb) is employed. Wireless terminals H through Lshown in FIG. 4, on the other hand, reside in advantageous communicationenvironments, so that a high speed transmission (fa) satisfies therequired reception quality. In such a case, transmission of the samemulticast information at the low rate and at the high rate results in ashorter transmission time for the high speed transmission and a longertransmission time for the low speed transmission as shown in FIG. 5.

[0066]FIG. 6 is a chart showing relationships in the frequency domainbetween the bandwidth after spreading, the bandwidth of high ratetransmission (fa) before spreading, and the bandwidth of low ratetransmission (fb) before spreading. As shown in FIG. 6, the bandwidth ofhigh rate transmission (fa) before spreading has a broader informationband than the bandwidth of low rate transmission before spreading.

[0067] Under the conditions that the bandwidth after spreading is B,information spreads over the bandwidth B during transmission no matterthe information is high rate information or low rate information. Aprocessing gain of spreading of the high rate information (fa) isdenoted as PGa, and a processing gain of spreading of the low rateinformation (fb) is denoted as PGb. Then, the rate of information, theprocessing gain, and the bandwidth B after spreading are related as:

High Rate Transmission: fa×PGa=B

Low Rate Transmission: fb×PGb=B

[0068] In this example, the processing gain of spreading is determinedin response to a transmission rate based on the above relationships.Since each wireless terminal has different reception quality because ofdifferent communication environment, the same multicast information istransmitted by using different processing gains as shown in FIG. 7. Thismakes it possible to receive multicast information in a short timeperiod if the wireless terminal is capable of receiving information athigh rate. If the wireless terminal receives the service in adisadvantaged communication environment such as an area of weak radioreception, the processing gain is raised to improve the transmissionquality of multicast information and a low rate reception is arranged.This results in a longer reception time, but the service can be properlyreceived.

[0069] The above description has been provided with reference to a casein which there are two reception qualities for wireless terminals thatreceive the same multicast service. It should be noted that there may bea larger number of reception qualities, and that multicast informationcan be transmitted in a multiplexed manner by using more than twoinformation rates and processing gains.

[0070] A wireless base station BS (which refers to the wireless basestation 20 hereinafter) sends the processing gain of spreading asdetermined above to a wireless terminal requesting a multicast serviceas part of notification of the transmission conditions of multicastinformation. The wireless terminal receives the multicast informationfrom the wireless base station BS by using the processing gain ofspreading as notified.

[0071] Parameters that control the transmission rate of multicastinformation delivered to a wireless terminal requesting the multicastinformation may be determined as part of the transmission conditions ofmulticast information based on the measured reception quality reportedby the wireless terminal.

[0072] In a system based on the CDMA scheme, the number of spreadingcodes is generally controlled to change the transmission rate. When amultiple access scheme based on use of a plurality of time slots isemployed, the number of spreading codes may be changed between differenttime slots. This makes it possible to deliver the same multicastinformation at different transmission rates.

[0073]FIG. 8 is an illustrative drawing showing the way the samemulticast information is delivered by changing the number of spreadingcodes. Here, the rate of multicast information as received from thenetwork NW is denoted as I, and it is assumed that this multicastinformation can be delivered at a transmission rate corresponding to therate of information I by using a timeslot and three spreading codes. Inthe example of FIG. 8, wireless terminals H through L positionedrelatively closer to the wireless base station BS have relatively betterreception quality, so that the multicast information can be transmittedat a relatively high rate (i.e., corresponding to the rate ofinformation I) to the wireless terminals H through L by using the threespreading codes C1, C2, and C3. This still maintains the service quality(i.e., reception quality) for the wireless terminals H through L above apredetermined level. Accordingly, use of the three spreading codes C1,C2, and C3 is chosen as the transmission conditions of multicastinformation for the wireless terminals H through L based on thereception quality thereof.

[0074] Since reception quality at the wireless terminals A through Efarther away from the wireless base station BS is poorer, use of thethree spreading codes for delivering multicast information to thewireless terminals A through E cannot achieve a service quality(reception quality) above the predetermined required level. In such acase, only two spreading codes may be used for the wireless terminals Athrough E that suffer a poor reception quality, thereby delivering themulticast information at a lower rate. This can maintain the servicequality at the wireless terminals A through E above the predeterminedlevel. Accordingly, use of two spreading codes C1 and C2 is chosen asthe transmission conditions of multicast information for the wirelessterminals A through E based on the reception quality thereof.

[0075] In this case, the wireless base station BS multiplexes andtransmits the multicast information spread by the three spreading codesC1, C2, and C3 and the multicast information spread by the two spreadingcodes C1 and C2 by including them in respective timeslots t(k) and t(k′)as shown in FIG. 9. The wireless base station BS notifies the wirelessterminals H through L of the timeslot t(k) and the three spreading codesC1, C2, and C3 as the transmission conditions of multicast information,and notifies the wireless terminals A through E of the timeslot t(k′)and the two spreading codes C1 and C2 as the transmission conditions ofmulticast information.

[0076] As a result, the wireless terminals H through L receive thedownlink timeslot t(k) from the wireless base station BS, and decode thereceived signal based on the three spreading codes C1, C2, and C3. Thewireless terminals A through E receive the downlink timeslot t(k′) fromthe wireless base station BS, and decode the received signal based onthe two spreading codes C1 and C2. This insures that all the wirelessterminals A through L receive the same delivery service of multicastinformation while maintaining the service quality above thepredetermined level.

[0077] In the above example, the number of spreading codes is chosenbased on the reception quality at each wireless terminal so as tocorrespond to one of the two different transmission rates. It should benoted that the number of spreading codes may be chosen so as to select atransmission rate from more than two different transmission rates. Insuch a case, the information delivery control unit 24 of the wirelessbase station BS may be provided with a table that lists receptionqualities and the corresponding numbers of spreading codes as shown inFIG. 10. In FIG. 10, reception quality level 5 (the highest quality)affords use of five spreading codes and transmission at transmissionrate T5 (the fastest rate). At reception quality level 4, four spreadingcodes are used, and transmission is possible at transmission rate T4(<T5). At reception quality level 3, three spreading codes are used, andtransmission is possible at transmission rate T3 (<T4). Further,reception quality level 2 permits use of two spreading codes andtransmission at transmission rate T2 (<T3). Finally, reception qualitylevel 1 (the lowest quality) allows only one spreading code to be usedfor transmission at transmission rate T1 (<T2). The information deliverycontrol unit 24 refers to the table as described above so as to choosethe number of spreading codes that corresponds to reception qualityreported from a wireless terminal.

[0078] The transmission rates listed in the table are related as:

T2=2×T1

T3=3×T1

T4=4×T1

T5=5×T1

[0079] In a system based on the time division multiple access, thenumber of timeslots is generally controlled to change the transmissionrate, and the positions of timeslots are changed to avoid co-channelinterference. In this example, the same multicast information is copiedand transmitted at once in the TDMA system that chooses the number oftimeslots in response to the rate of multicast information.

[0080]FIG. 11 is an illustrative drawing showing the way the samemulticast information is transmitted by changing the position oftimeslots. The rate of multicast information is denoted as I, and it isassumed that the information of the rate I can be transmitted by usingthree timeslots. Wireless terminals H through L shown in FIG. 11 aresupposed to satisfy required service quality when transmission iscarried out as assumed above. Namely, the wireless base station BStransmits multicast information by using a timeslot group t(k) comprisedof three timeslots as shown in FIG. 12A, and each of the wirelessterminals H through L can satisfy the required service quality whenreceiving these timeslots.

[0081] Wireless terminals A through E shown in FIG. 11 cannot properlyreceive the information conveyed by the timeslot group t(k) because ofco-channel interference, so that these terminals cannot satisfy therequired service quality if transmission is carried out as assumed. Insuch a case, the wireless base station BS uses a timeslot group t(k′)for the wireless terminals A through E concurrently with thetransmission of multicast information to other terminals by use of thetimeslot group t(k). In this manner, all the wireless terminals Athrough L can receive the same multicast services with satisfactoryservice quality.

[0082] In this example, the timeslot group t(k′) has two availabletimeslots. As the number of timeslots decreases from three to two, thetransmission rate also decreases.

[0083] Further, the modulation scheme (the modulation multi-number) canbe changed to control the transmission rate irrespective of whatmultiple access scheme is employed. Based on this, delivering the samemulticast information that is modulated by different modulation schemes(different modulation multi-numbers) in different timeslots makes itpossible to achieve different transmission rates of the same multicastinformation.

[0084]FIG. 13 is an illustrative drawing showing the way the samemulticast information is transmitted by changing the modulationmulti-number. The rate of multicast information as received from thenetwork NW is denoted as I, and it is assumed that this multicastinformation can be delivered at a transmission rate corresponding to therate of information I by modulating the multicast information based onthe 16 QAM modulation scheme. In the example of FIG. 13, wirelessterminals H through L positioned relatively closer to the wireless basestation BS have relatively better reception quality, so that themulticast information can be transmitted at a relatively high rate(i.e., corresponding to the rate of information I) to the wirelessterminals H through L by using the 16 QAM modulation scheme. This stillmaintains the service quality (i.e., reception quality) for the wirelessterminals H through L above a predetermined level. Accordingly, use ofthe 16 QAM modulation scheme is chosen as the transmission conditions ofmulticast information for the wireless terminals H through L based onthe reception quality thereof.

[0085] Since reception quality at the wireless terminals A through Efarther away from the wireless base station BS is poorer, use of the 16QAM modulation scheme for modulating and transmitting multicastinformation to the wireless terminals A through E cannot achieve aservice quality (reception quality) above the predetermined requiredlevel. In such a case, the BPSK modulation scheme may be used for thewireless terminals A through E that suffer a poor reception quality,thereby delivering the multicast information at a lower rate. This canmaintain the service quality at the wireless terminals A through E abovethe predetermined level. Accordingly, use of the BPSK modulation schemeis chosen as the transmission conditions of multicast information forthe wireless terminals A through E based on the reception qualitythereof.

[0086] In this case, the wireless base station BS transmits themulticast information modulated according to 16 QAM modulation schemeand the multicast information modulated according to the BPSK modulationscheme by including them in respective timeslots t(k) and t(k′) as shownin FIG. 14. The wireless base station BS notifies the wireless terminalsH through L of the timeslot t(k) and the 16 QAM modulation scheme as thetransmission conditions of multicast information, and notifies thewireless terminals A through E of the timeslot t(k′) and the BPSKmodulation scheme as the transmission conditions of multicastinformation.

[0087] As a result, the wireless terminals H through L receive thedownlink timeslot t(k) from the wireless base station BS, and decode thereceived signal based on the demodulation scheme corresponding to the 16QAM modulation scheme. The wireless terminals A through E receive thedownlink timeslot t(k′) from the wireless base station BS, and decodethe received signal based on the demodulation scheme corresponding tothe BPSK modulation scheme. This insures that all the wireless terminalsA through L receive the same delivery service of multicast informationwhile maintaining the service quality above the predetermined level.

[0088] In the above example, a modulation scheme (modulationmulti-number) is chosen based on the reception quality at each wirelessterminal so as to correspond to one of the two different transmissionrates. It should be noted that a modulation scheme may be chosen so asto select a transmission rate from more than two different transmissionrates. In such a case, the information delivery control unit 24 of thewireless base station BS may be provided with a table that listsreception qualities and the corresponding modulation schemes (modulationmulti-numbers) as shown in FIG. 15. In FIG. 15, reception quality level5 (the highest quality) affords use of the 256 QAM modulation schemehaving the largest modulation multi-number and transmission attransmission rate T5 (the fastest rate). At reception quality level 4,the 64 QAM modulation scheme is used, and transmission is possible attransmission rate T4 (<T5). At reception quality level 3, the 16 QAMmodulation scheme is used, and transmission is possible at transmissionrate T3 (<T4). Further, reception quality level 2 permits use of theQPSK modulation scheme and transmission at transmission rate T2 (<T3).Finally, reception quality level 1 allows the BPSK modulation scheme tobe used for transmission at transmission rate T1 (<T2). The informationdelivery control unit 24 refers to the table as described above so as tochoose a modulation scheme (modulation multi-number) that corresponds toreception quality reported from a wireless terminal.

[0089] The transmission rates listed in the table are related as:

T2=2×T1

T3=2×T2

T4=2×T3

T5=2×T4

[0090] In the above example, the modulation scheme (modulationmulti-number) is changed so as to conform to the reception quality ateach wireless terminal. Alternatively, the bit rate of multicastinformation may be controlled.

[0091] The wireless base station BS receives multicast information atthe information rate I from the network NW, and stores the multicastinformation successively in the multicast information storage unit(buffer) 22. Then, the wireless base station BS successively transmitsthe multicast information stored in the multicast information storageunit 22 by using different transmission conditions (e.g., usingdifferent transmission rates or the like) as described above. When thesame multicast information is transmitted to a plurality of wirelessterminals at different transmission rates, an attempt to arrange thesame delivery time will fail since the multicast information transmittedat a low transmission rate cannot be timely finished. When the samedelivery time is desired, therefore, it is desirable to compress themulticast information directed to each wireless terminal in accordancewith the transmission rate thereof. This insures the timely completionof multicast information delivered at a low transmission rate.

[0092] As previously described, the information delivery control unit 24chooses the transmission rate of multicast information (i.e., the numberof spreading codes, the number of timeslots, the modulationmulti-number, the bit rate, etc.) based on the reception quality of eachwireless terminal. When this is done, the information delivery controlunit 24 compresses the multicast information in accordance with theselected transmission rate. A description will be given in the followingwith regard to the compression rate CR and how it is chosen based on theinformation rate I of multicast information as received from the networkNW and the transmission rate T determined as a proper transmission rate.

[0093] When the information rate I of multicast information is no largerthan the transmission rate T (I≦T: i.e., high speed transmission), thecompression rate R is set equal to “1”. Namely, no compression ofmulticast information is performed. When the information rate I ofmulticast information is larger than the transmission rate T (I>T: i.e.,low speed transmission), the compression rate R is set as:

CR=1−(I−T)/I.

[0094] The information delivery control unit 24 carries out apredetermined algorithm to compress multicast information stored in themulticast information storage unit 22 so as to achieve the selectedcompression rate CR. The multicast information compressed in this manneris then delivered from the transceiver 21 to the wireless terminals.

[0095] In this case, the wireless base station BS notifies the wirelessterminals of the compression rate CR as one of the transmissionconditions of multicast information. In a given wireless terminal thatreceives the multicast information at a given transmission rate, thecontrol unit 13 decompresses the received multicast information by usinga decompression algorithm corresponding to the compression algorithm,thereby producing the original multicast information.

[0096] When an optimum compression rate needs to be selected from aplurality of pre-selected compression rates, the compression rate may becomputed as described above based on the information rate I of multicastinformation and the transmission rate T, and an optimum compression ratemay be chosen from the pre-selected compression rates by selecting onethat is the smallest among those larger than the computed compressionrate.

[0097] There is a case in which multicast information provided from theinformation source (server) to the wireless base station through thenetwork includes coded still images or a series of coded still images.In such a case, information may be discarded once at predeterminedintervals to reduce the size of information, thereby making it possibleto transmit the multicast information at a lower transmission rate. Inthis case, the rate of information reduction is the same as thecompression rate described above. If the reduction rate needs to be setequal to ⅘, for example, one in every five image frames is discardedduring transmission to wireless terminals.

[0098] When multicast information obtained at the rate I from thenetwork NW is transmitted to a plurality of wireless terminals atdifferent transmission rates, without adjustment of information size,timing to read information from the multicast information storage unit22 will be slower the slower the transmission rate becomes. Eachtransmission rate needs to be adjusted to make sure that such delay doesnot adversely affect other processing of the wireless base station BS.

[0099] As shown in FIG. 16, multicast information obtained from thenetwork NW is temporarily stored in the multicast information storageunit (buffer) 22. Since this stored multicast information is to betransmitted at different transmission rates, a point at which the storedmulticast information is read from the multicast information storageunit 22 differs depending on the transmission rates. For example, theremay be a transmission rate T and a transmission rate T′, which are usedby the wireless base station BS to transmit multicast information. Insuch a case, the buffer is provided with pointers corresponding to thetransmission rates. The transmission rate T is larger than thetransmission rate T′ (T>T′), and a pointer 1 is moved through the bufferto read multicast information that is transmitted at the transmissionrate T′ through a given channel, with a pointer 2 being moved throughthe buffer to read multicast information that is transmitted at thetransmission rate T through another channel.

[0100] Information stored in the buffer area already swept by thepointer 1 can be cleared. Assuming that the transmission rates T and T′represent average rates, an average delay time can be obtained based onan average D of the amount of information residing between the pointer 2and the pointer 1. That is, the average delay time is obtained as:

Average Delay Time=D/(T−T′)

[0101] The average delay time as computed above should not affect theperformance of applications in the wireless base station BS. To insurethis, the information delivery control unit 24 adjusts the transmissionrates T and T′.

[0102] In the above example, each wireless terminal measures receptionquality, and reports the measured results to the wireless base stationBS. Based on the measured results of the reception quality reported fromeach wireless terminal, the wireless base station BS determines thetransmission conditions of multicast information, and, then, notifieseach wireless terminal of the transmission conditions such as theprocessing gain of spreading, the number of spreading codes, theidentification of timeslots, the modulation scheme (modulationmulti-number), the compression rate of information, etc. Then, the samemulticast information is transmitted according to the transmissionconditions as determined. Each wireless terminal receives the multicastinformation in accordance with the transmission conditions of themulticast information. This makes it possible for each wireless terminalto receive the multicast information having sufficiently high qualityeven if communication environments vary between the wireless terminals.

[0103] The above example has been described with reference to a case inwhich each wireless terminal measures the reception quality as anindication of communication environment, and notifies the wireless basestation BS. Alternatively, such notification can be omitted. Forexample, the wireless base station BS transmits the same multicastinformation by using different sets of transmission conditions (i.e.,different transmission rates, different timeslot positions, etc.), andnotifies each wireless terminal of the multiple sets of transmissionconditions by using a perch channel. Each wireless terminal selects oneof the multiple sets of transmission conditions such that the selectedset of conditions provide the highest service quality within thecapacity of the measured reception quality. Each wireless terminalreceives the multicast information in such a manner as to conform to theselected set of transmission conditions.

[0104] As described above, the system may be configured such that eachwireless terminal does not need to inform the wireless base station BSof the measured results of the reception quality indicating thecommunication environments. In order to achieve such a configuration,each wireless terminal needs to be provided with the function ofselecting an optimum transmission condition in response to the measuredresults.

[0105] Further, the present invention is not limited to theseembodiments, but various variations and modifications may be madewithout departing from the scope of the present invention.

[0106] The present application is based on Japanese priority applicationNo. 2000-140845 filed on May 12, 2000, with the Japanese Patent Office,the entire contents of which are hereby incorporated by reference.

What is Claimed is
 1. A method of providing a multicast service from aninformation delivery apparatus to wireless terminals, comprising thesteps of: transmitting, from the information delivery apparatus, aplurality of sets of multicast information, said sets being identical toeach other as to contents thereof but differing in transmissionconditions; and receiving, at any given one of the wireless terminals,one of the sets of multicast information being transmitted under one ofthe differing transmission conditions.
 2. The method as claimed in claim1, wherein the differing transmission conditions include differingtransmission rates at which the multicast information is transmitted. 3.The method as claimed in claim 2, wherein communication between theinformation delivery apparatus and the wireless terminal is based oncode division multiple access, and the differing transmission ratesdiffer in a number of spreading codes used in the transmission ofmulticast information.
 4. The method as claimed in claim 2, whereincommunication between the information delivery apparatus and thewireless terminal is based on time division multiple access, and thediffering transmission rates differ in a number of timeslots used in thetransmission of multicast information.
 5. The method as claimed in claim2, wherein the differing transmission rates differ in a number ofmodulation levels used for modulating the multicast information.
 6. Themethod as claimed in claim 2, wherein the differing transmission ratesdiffer in a transmission bit rate of the multicast information.
 7. Themethod as claimed in claim 1, wherein communication between theinformation delivery apparatus and the wireless terminal is based oncode division multiple access, and the differing transmission conditionsinclude differing processing gains of spreading the multicastinformation.
 8. The method as claimed in claim 1, wherein the differingtransmission conditions include differing positions of timeslots used inthe transmission of multicast information.
 9. The method as claimed inclaim 1, further comprising the steps of: measuring reception quality ateach of the wireless terminals, and notifying the information deliveryapparatus of measured results of the reception quality; and determining,at the information delivery apparatus, the differing transmissionconditions based on the measured results of the reception quality, thediffering transmission conditions being used to transmit the pluralityof sets of multicast information.
 10. The method as claimed in claim 9,further comprising a step of having the information delivery apparatusnotify the wireless terminals of the differing transmission conditions,wherein said step of receiving receives the one of the sets of multicastinformation by using the one of the differing transmission conditionsthat is notified by the information delivery apparatus.
 11. The methodas claimed in claim 1, further comprising the steps of: transmitting,from the information delivery apparatus to the wireless terminals, thediffering transmission conditions used to transmit the plurality of setsof multicast information; and measuring reception quality at each of thewireless terminals, and selecting a transmission condition from thereported differing transmission conditions based on the measuredreception quality, the selected transmission condition being used forreceiving one of the sets of multicast information.
 12. The method asclaimed in claim 2, further comprising a step of decreasing a size ofthe multicast information to be transmitted as the differingtransmission rates decrease.
 13. The method as claimed in claim 12,wherein said step of decreasing adjusts a compression rate of themulticast information to be transmitted so as to decrease the size ofthe multicast information.
 14. The method as claimed in claim 2, furthercomprising the steps of: storing the multicast information in a bufferat the information delivery apparatus as the multicast information isreceived from a network; and assigning channels to the respective setsof the multicast information as the respective sets are read from thebuffer at rates of reading corresponding to the differing transmissionrates.
 15. The method as claimed in claim 14, further comprising a stepof adjusting the differing transmission rates based on delays of thereading of the multicast information from the buffer.
 16. An informationdelivery apparatus for delivering multicast information to wirelessterminals through wireless routes, comprising: a multicast informationstorage unit which stores the multicast information to be transmitted;an information delivery control unit which transmits a plurality of setsof the multicast information, which are identical to each other as tocontents thereof but differ in transmission conditions.
 17. Theapparatus as claimed in claim 16, wherein the differing transmissionconditions include differing transmission rates at which the multicastinformation is transmitted.
 18. The apparatus as claimed in claim 17,wherein communication between said information delivery apparatus andthe wireless terminal is based on code division multiple access, and thediffering transmission rates differ in a number of spreading codes usedin the transmission of multicast information.
 19. The apparatus asclaimed in claim 17, wherein communication between the informationdelivery apparatus and the wireless terminal is based on time divisionmultiple access, and the differing transmission rates differ in a numberof timeslots used in the transmission of multicast information.
 20. Theapparatus as claimed in claim 17, wherein the differing transmissionrates differ in a number of modulation levels used for modulating themulticast information.
 21. The apparatus as claimed in claim 17, whereinthe differing transmission rates differ in a transmission bit rate ofthe multicast information.
 22. The apparatus as claimed in claim 16,wherein communication between said information delivery apparatus andthe wireless terminal is based on code division multiple access, and thediffering transmission conditions include differing processing gains ofspreading the multicast information.
 23. The apparatus as claimed inclaim 16, wherein the differing transmission conditions includediffering positions of timeslots used in the transmission of multicastinformation.
 24. The apparatus as claimed in claim 16, wherein saidinformation delivery control unit determines the differing transmissionconditions based on reception qualities of the wireless terminalsreported from the wireless terminals, the differing transmissionconditions being used to transmit the plurality of sets of multicastinformation.
 25. The apparatus as claimed in claim 24, wherein saidinformation delivery control unit notifies the wireless terminals of thedetermined differing transmission conditions.
 26. The apparatus asclaimed in claim 16, wherein said information delivery control unitnotifies the wireless terminals of the differing transmission conditionsused to transmit the plurality of sets of multicast information.
 27. Theapparatus as claimed in claim 17 wherein said information deliverycontrol unit decreases a size of the multicast information to betransmitted as the differing transmission rates decrease.
 28. Theapparatus as claimed in claim 27 wherein said information deliverycontrol unit adjusts a compression rate of the multicast information tobe transmitted so as to decrease the size of the multicast information.29. The apparatus as claimed in claim 17, wherein said informationdelivery control unit assigns channels to the respective sets of themulticast information as the respective sets are read from saidmulticast information storage unit at rates of reading corresponding tothe differing transmission rates.
 30. The apparatus as claimed in claim29, wherein said information delivery control unit adjusts the differingtransmission rates based on delays of the reading of the multicastinformation from said multicast information storage unit.
 31. A wirelessterminal for receiving multicast information from an informationdelivery apparatus through wireless routes, comprising a control unitwhich measures reception quality of signals received from theinformation delivery apparatus, and receives one of sets of themulticast information sent from the information delivery apparatus byusing transmission conditions selected based on the measured receptionquality, wherein the sets of multicast information are identical to eachother but differ in transmission conditions.
 32. The wireless terminal,wherein said control unit notifies the information delivery apparatus ofthe measured reception quality, and is notified by the informationdelivery apparatus of the transmission conditions that are to be usedfor receiving the one of the sets of the multicast information sent fromthe information delivery apparatus.